1. Field of the Invention
The present invention pertains to apparatus for polishing relatively thin workpieces and, in particular, to the chemical/mechanical polishing of semiconductor wafers.
2. Description of the Related Art
In the fabrication of semiconductor devices, the devices are typically mass produced by stacking layers of device structures on the surface of a semiconductor wafer. With the addition of each layer, the wafer must undergo surface treatment using chemical/mechanical polishing (CMP) or other processes in preparation for fabrication of the next wafer layer. A wafer carrier is used to acquire and provide backing support for the wafer as the wafer surface is pressed against a polish pad or other working surface, such as a linear belt.
Typically, surface treatment operations are concerned with restoring or maintaining wafer flatness, and many advantages have been a achieved in meeting these objectives. However, further advantages are continually being sought. For example, it is important that the wafer carrier be able to take on various angular positions with respect to the plane of the wafer surface being treated. Accordingly, wafer carriers are provided with some form of gimbal mechanism which typically includes a number of cooperating mechanical components. However, such mechanical gimbal arrangements typically vary somewhat in their freedom of movement from one angular position on the wafer carrier to another. Further, mechanical gimbal arrangements are susceptible to corrosion and contamination, requiring the wafer carrier to be disassembled for repair and replacement of deteriorated components.
During semiconductor wafer polishing, a downforce and reciprocating motion is typically applied to the wafer carrier, and these applied forces may alter the freedom of movement of the gimbal action. Over the life of the wafer carrier, the mechanical gimbal components are susceptible to ongoing wear, which, in precision wafer polishing, can interfere with desired precision polishing results. Further, because of the mechanical hysteresis inherent in mechanical gimbal actions, the effects exhibited on polished wafers can vary so as to complicate diagnostic or trouble shooting efforts.
Restrictions in the freedom of movement of the gimbal action may influence the uniformity of a planarized wafer surface. With ongoing industry demands to increase circuit density, continual improvements in gimbal action are being sought. Wafer carriers must meet certain practical demands, one of which is their ability to produce a highly planar surface that is uniform across usable portions of the wafer being treated. Increasingly, wafer carriers and other components of wafer surface treatment processes are being called upon to produce highly planar surfaces across the substantial entirety of the wafer. This places considerable demands on the gimbal action of the wafer carrier.
In order to reduce the cost of ownership of a wafer carrier, it is desirable to avoid complicated gimbal actions having a relatively large number of cooperating parts, especially mechanical parts which are subject to ongoing degradation due to wear and contamination.
Extension rings protrude from a backing pad to confine a wafer being pressed by the backing pad, while allowing a controlled degree of movement of the wafer across the backing pad surface. Typically, polish surfaces are compressible to some extent, and are deformed by wafer down pressure, with the wafer "sinking" into the polish surface and with the retaining ring coming closer to the polish surface. It is desirable to avoid contacting the polish surface with the extension ring, as this may alter the polishing surface, so as to render the polishing operation unpredictable.